1. The Field of the Invention
This invention relates to devices and methods for use in connection with electrochemical analyses and determinations and, more particularly, to novel devices and methods which are capable of being used in liquid or gaseous environments to induce, detect, and monitor electrochemical oxidation/reduction reactions.
2. The Prior Art
It is a well-known principle of science that the atoms and molecules of most substances, except air and a number of pure solvents, will undergo an electrochemical reaction in the presence of a strong electric field. This electrochemical reaction is characterized by the atom or molecule either losing or gaining one or more electrons in a process known as ionization. If the atom or molecule loses one or more electrons in the electrochemical reaction, it is said to have been "oxidized." If, on the other hand, the atom or molecule gains one or more electrons during the electrochemical reaction, it is said to have been "reduced."
For example, a chlorine molecule is composed of two chlorine atoms and is electrically neutral. In other words, the chlorine molecule does not naturally have an associated electrical charge. In the presence of a strong electric field, however, the chlorine molecule will be reduced by gaining two electrons. The chlorine molecule is thereby transformed into two chloride ions, each of which is negatively charged.
The analysis and detection of electrochemicaloxidation or reduction reactions has many important uses. Many forms of chemical analysis are, for example, dependent upon the accurate detection and quantitative measurement of electrochemical reactions. In addition, the study of electrochemical reactions has several important uses in research and development activities.
For example, the magnitude of the electric field which is required in order to cause a particular atom or molecule to undergo oxidation or reduction varies with each chemical compound. Thus, if one can measure the magnitude of the electric field at which a compound is oxidized or reduced, it is often possible to identify the particular compound.
In addition to determining the identity of a substance which is being oxidized or reduced, the electrochemical reaction can be monitored over time so as to determine the concentration of the material which is undergoing the reaction. This follows from the fact that the rate at which atoms or molecules of a material are oxidized or reduced is proportional to the concentration of the material in the environment.
To determine the identity and concentration of a particular material may, of course, be important in several contexts. For example, it may be desirable to identify impurities in a mixture, such as, for example, in a public water supply. It may also be important to detect the presence of certain harmful substances and determine the concentration of such substances. The monitoring and analysis of electrochemical reactions is, thus, often used for many of these purposes.
In the course of research and development, it is often useful to study the oxidation and reduction of various substances. For example, electric batteries use oxidation and reduction reactions to supply an electric current. The amount of current which can be delivered by the battery is limited by the speed at which the materials within the battery can be oxidized and reduced. Thus, in an effort to develop better, higher current density batteries, it would be important to identify those materials which are oxidized and reduced most rapidly.
The magnitude of the electric field which is required to cause an atom or molecule to undergo either oxidation or reduction is very substantial. Typically, the magnitude of this electric field must be in the order of between 100 million (10.sup.8) volts per centimeter to one billion (10.sup.9) volts per centimeter. In other words, if two electrodes are placed one centimeter apart in a vacuum, the electric potential difference between the two electrodes would need to be between 100 million volts and one billion volts in order to create an electric field of sufficient strength to cause an atom or molecule to undergo oxidation or reduction.
Obviously, it is both unsafe and uneconomical in mist cases to provide the magnitude of electric field required for oxidation or reduction by simply providing two electrodes with a large difference in electric potential. Accordingly, those skilled in the art have conventionally used a device called an electrochemical cell.
A typical electrochemical cell comprises two electrodes which are immersed in a solvent, such as, for example, water. An ionic substance (commonly called the "electrolyte") is then dissolved in the solvent. The ionic substance is one which dissociates upon being dissolved in a solvent into charged atoms or molecules which are called "ions." This is, for example, a characteristic of virtually all salts. Thus, if water is used as a solvent in an electrochemical cell, a suitable salt, such as, for example, sodium chloride ("NaCl"), may be dissolved in the water to serve as the electrolyte.
In operation, an electric potential is imposed between the two electrodes in the cell solution. One of the electrodes will, of course, have a relative positive charge, and the other electrode will have a negative charge. These charged electrodes in turn attract the ions in the solution which have the opposite charge. Thus, the positively charged electrode will attract negatively charged ions and vice versa.
After a brief time period, a sufficient number of ions will have collected adjacent each electrode so as to form a compact and diffuse charged layer of ions. Provided there is a sufficient amount of electrolyte in the solution, the layer of ions will have a total charge which is equal in magnitude, but opposite in polarization, to the charge of the electrode. The ion layer thus balances the charge of the electrode so that the electrode does not have any significant effect on particles within the electrochemical cells which are located a great distance from the electrode. In other words, the electrode becomes electrically isolated from the solution by means of the charged ion layer.
The charged ion layer is typically located approximately one to three angstroms (100 millionths of a centimeter) away from the electrode. The charged ion layer thus simulates a second oppositely charged electrode positioned parallel to the electrode with a very small distance therebetween. The region between the real electrode and the imaginary, parallel electrode is often referred to as the electrical "double layer."
As previously indicated, the magnitude of an electric field may be expressed as the difference in electric potential per unit distance. Using this definition, it can be readily appreciated that a small difference in potential between the electrode and the solution in the electrochemical cell will create a very high magnitude electric field in the region of the double layer described above.
For example, there may be a one volt electric potential difference between the electrode and the solution in the electrochemical cell. From the foregoing discussion, it will be appreciated that this entire change in potential takes place over the distance of the double layer surrounding the electrode.
Further, as previously indicated, the double layer is typically on the order of from one to three angstroms thick. Thus, even a potential difference of one volt between the electrode and the solution in the cell will create an electric field having a magnitude on the order of 100 million volts per centimeter in the region of the double layer. A potential difference of one volt is, of course, easy to obtain. Accordingly, it is quite feasible to create in the double layer region of an electrochemical cell an electric field having a sufficient magnitude to cause a reduction/oxidation ("redox") reaction.
To illustrate this, suppose an electrochemical cell contained a number of benzene molecules. As benzene molecules enter a "positive" double layer region adjacent the electrodes, the molecules would be in an electric field of sufficient magnitude to cause the benzene molecules to lose an electron and undergo an oxidation reaction.
Importantly, the configuration of the electrochemical cell is readily adapted to monitor redox reactions and detect that they are occurring. In the foregoing example, the benzene molecules would lose an electron, as already noted. At the same time, an atom or molecule adjacent the other electrode in the electrochemical cell would gain an electron, and a current would, therefore, flow through the circuit connecting the two electrodes. Accordingly, by monitoring the current in the circuit between the two electrodes, one can easily detect and monitor the redox reaction.
In using an electrochemical cell of the type described above for purposes of analysis, one might gradually increase the potential difference between the electrode and the solution until a current is detected in the external circuit, thereby indicating that a redox reaction is taking place. This potential difference is referred to as the "redox potential" of the substance which is undergoing the redox reaction. Fortunately, redox potentials are well documented. Therefore, after determining the redox potential of the substance, one may be able to identify the substance which is in the electrochemical cell.
In addition, the magnitude of the current in the external circuit between the two electrodes is an indication of the frequency at which redox reactions are taking place. The number of redox reactions which take place during a given period of time is an indication of the concentration of the substance undergoing the redox reaction. Accordingly, by monitoring the current in the external circuit, one may also determine the concentration of the substance within the electrochemical cell.
The typical, elementary electrochemical cell described above is easy to construct and use. Nevertheless, there are a number of obstacles to overcome in using such an electrochemical cell to obtain accurate results.
The first significant obstacle arises from the fact that it is theoretically impossible to measure the potential of a single electrode; only differences in potential can be measured. The potential is then usually measured against a standard reference electrode. If the reference electrode is subjected to large currents, however, such as those found in the elementary two-electrode electrochemical cells described above, the standard potential of the reference electrode will change.
In order to correct this situation, those skilled in the art typically make one of the electrodes (the "working electrode") very small, while making the other electrode (the "reference electrode") massive in comparison. In this way, the current per unit area is kept quite small, and the reference electrode potential does not significantly change.
Often, however, even small changes in the reference electrode potential are unacceptable. In these cases, it then becomes necessary to use a third electrode which is not a part of the electrochemical circuit as a reference electrode. The use of such a third electrode, however, complicates the electrical circuitry and makes the system somewhat more difficult to calibrate and use.
Additionally, since the error in making the potential measurements is directly proportional to the current through the system, high currents in the system also lead to a high degree of error in measuring the potential. Moreover, the error in potential measurement is also directly proportional to the resistance of the system, and the resistance of the system is typically quite high. This situation compounds thus the problem and makes the error in measuring potential still higher.
In an effort to reduce the error in potential measurement caused by the high current, those skilled in the art typically try to reduce the resistance of the system. This is done by adding a large amount of the electrolyte which makes the solution of the electrochemical cell more electrically conductive. By increasing the conductivity of the solution, and thereby reducing its resistance, it is hoped that the potential error can be brought down to acceptable limits even in the presence of relatively large currents.
In addition to the relatively large currents in the system which have already been mentioned, there is a large capacitive current associated with the movement of ions in the solution toward the two electrodes to develop the double layer. This current is typically very large and, in some cases, can totally drown out the current caused by the redox reaction which one is attempting to measure.
To reduce the difficulties caused by the large capacitive current, those skilled in the art again relay upon the use of large amounts of electrolyte. By introducing a large number of charged ions into the electrochemical cell, the ions do not have to move far in the solution before the working electrode is properly balanced by an ionic layer. Accordingly, the working electrode can be electrically isolated quickly, thereby reducing the interference of the capacitive current on the measurement being made.
An additional difficulty with conventional electrochemical cells arises from the mechanism by which the redox reaction proceeds over time. As the electrochemical cell is first energized at the appropriate potential, atoms and molecules which are immediately adjacent the working electrode are quickly depleted from the double layer region as they undergo a redox reaction. After this initial surge of activity, one must wait until additional atoms and molecules can diffuse through the electrochemical cell toward the working electrode. As a typical result, the reading from an electrochemical cell stepped between two potentials shows an initial surge of high current which thereafter drops down to a lower current reading.
In many cases, the initial transient high current reading may interfere with the analysis being done, and it may be desirable to maintain a steady state throughout the reaction. Accordingly, since the rate of the redox reaction depends somewhat on the voltage drop across the double layer, it is typical to approach a steady state situation by decreasing the speed at which the voltage across the double layer is changed over the course of a given analysis. In this way, it is hoped that the readings can approach a steady state condition at virtually all voltage levels.
A somewhat more troublesome consequence of the rapid depletion of atoms and molecules from the double layer is that it becomes difficult to study the kinetic limit of the reaction. The kinetic limit of the redox reaction is the maximum speed at which the redox reaction can occur if there is an unlimited supply of the substance being reduced or oxidized. As mentioned previously, studying the kinetic limits of particular reactions is important in many research and development applications, such as, in the development of high current density power supplies.
Unfortunately, in conventional electrochemical cells, the rate at which the redox reaction takes place is not limited by the actual speed of the reduction but is, rather, limited by the speed at which atoms and molecules can diffuse through the electrochemical cell solution. This situation is worsened by the fact that most electrodes are essentially planar and atoms and molecules can, therefore, only approach the electrode from one direction. This further retards the diffusion rate and limits the speed of the redox reaction.
Some attempts to approach the kinetic limits of a reaction have been made by changing the geometry of the electrode. For example, if the electrode is shaped spherically, atoms and molecules can approach the electrode from virtually any direction. This results in the atoms and molecules diffusing toward the electrode at a much higher rate.
In addition, when attempting to study the kinetics of the reaction, it has become quite common to use rotating or otherwise agitated electrodes. By rotating the electrode through the electrochemical cell, one is not limited solely by diffusion, since the movement of the electrode itself serves to bring atoms and molecules into contact with the working electrode.
In summary, while conventional electrochemical cells are adequate for many applications, there use in some situations of prime interest is very complex. Not only can it be difficult to set up the electrochemical cell with all of the required compensations for error, but the complexity of the system also increases the probability of some human errors. Accordingly, those skilled in the art have attempted to build electrochemical cells which are simpler in construction and which avoid the disadvantages set forth above.
One promising developing in recent years has been the use of a very small electrode as the working electrode in an electrochemical cell. These very tiny electrodes have come to be known as "microelectrodes."
Microelectrodes draw a very small current. As a result, the error in measuring the potential in the system is reduced. Consequently, it may often not be necessary to use a third reference electrode when making electrochemical measurements.
Also, since the size of the working microelectrode is extremely small, only a very few ions are needed to electrically isolate the electrode in the electrochemical call. Accordingly, there is a much smaller capacitive current in the electrochemical call to interfere with accurate readings.
Further, as the microelectrode approaches atomic proportions, the electrode begins to act much more like a hemisphere-shaped electrode. Accordingly, atoms and molecules are able to diffuse toward the electrode from several different directions, thereby decreasing the limitation caused by the rate of diffusion.
In spite of these advantages, however, electrochemical cells which are currently being used suffer from a number of significant drawbacks. First, all of the electrochemical cells which are currently in use require some type of liquid solvent in which an electrolyte is dissolved. This, of course, limits the application of the electrochemical cell to the liquid phase.
Similarly, the structure and configuration of prior art electrochemical cells with their liquid solvents and electrolytes virtually eliminates many potential uses of electrochemical cells. For example, electrochemical cells cannot presently be used to test a substance in a gaseous environment. Rather, the substance to be tested must typically be sampled and then returned to the laboratory for analysis.